Composite filter and method of making the same

ABSTRACT

The invention refers to a composite filter for filtering a stream of ambient air comprising at least one non-prebonded upstream tier and one non-prebonded downstream tier, wherein the ratio of absolute pore volume of upstream tier to downstream tier RAPV&gt;2, and the absolute projected fiber coverage of upstream tier and of downstream tier APFC&gt;95%.  
     Further, the invention refers to a method of making such a composite filter comprising the, steps of  
     (a) laying down a filtration material onto a support to form the upstream non-prebonded tier,  
     (b) depositing onto the upstream tier the downstream non-prebonded tier, and  
     (c) bonding the tiers to form a composite filter having a unitary stratified structure.

FIELD OF THE INVENTION

[0001] The invention relates, to a composite filter for removing solidparticles entrained in a stream of ambient air. More specifically, itrelates to a composite filter comprising at least one non-prebondedupstream tier and one non-prebonded downstream tier useful for filteringparticulates from ambient air.

[0002] The term “pre-bonded” means herein that a composition of filtermedium, such as thermally bondable fusing fibers or adhesively bindablefibers, is treated in a manner effective to activate the bindingmechanism thereby forming a separate, free-standing, cohesive, andtypically self-supporting web of that filter composition. Such apre-bonded web can be mechanically manipulated by such processes aswinding on a roll, unwinding from a roll, cutting and the like.

[0003] The term “tier” herein means a band formed from non-prebondedfilter material into a stratum of unitary stratified structure. Incontrast, a “layer” means a separately, prebonded, self-supporting webof filter material.

BACKGROUND AND PRIOR ART

[0004] In recent times, the technology for filtering particulates fromgases has become quite sophisticated in both commonplace applicationssuch as consumer oriented vacuum cleaning of dirt and dust as well asvery demanding industrial applications such as removal from gases ofspecific particle size fractions of wide varieties of contaminantsincluding from inert to biochemically sensitive, among others. It is nowwell appreciated that the contaminating particulates in a gas stream canhave a wide variety of sizes, geometric shapes, e.g., elongated andspherical, and chemical and physical compositions, e.g., odor-free andodoremitting particles.

[0005] Consequently, filtration technology has evolved to provide filtermedia which are adapted to optimally filter specific fractions of thecontaminating particulates. Also, this technology has developedtechniques for optimizing various performance characteristics of filterssuch as maintaining low pressure drop across the filter and increasingthe filter service life so as to extend the length of time betweenfilter element replacements.

[0006] The traditional approach to achieving these objectives has beento provide a multilayer filter medium composed of separate, individuallydesigned layers which are each intended to accomplish primarily one, andsometimes several specific filter functions. For example, a very open,porous and thin scrim is often used to protect underlying filter layersfrom abrasion by fast moving, large and hard particles; a porous andbulky layer is typically used to capture substantial amounts of chieflylarge particles, and an ultrafine diameter filament, low porosity layeris usually prescribed for removing the smallest particles to increasefiltration efficiency. From the many choices available, separate filterlayers are selected and combined in a preselected sequence thenassembled as a group to form a multilayer, and therefore multifunctionalfilter. The one or more adjacent layers can be bonded to each other orthe layers can be unbonded. Optionally, the individual layers can besandwiched between covers, typically of paper, for structural integrityand ease of handling.

[0007] A drawback of the aforementioned multilayer system ofconstructing multifunctional filters is that there is repetitiveprocessing of the filter media which can be excessive. That is, thefilter material in a given layer is first processed to form theindividual layer, then it is processed to assemble that layer in themultilayer filter. Each step adds to the compaction and cover, if everslight, of the final filter product. This tends to raise the pressuredrop through the filter and reduce dust holding capacity, therebylimiting service life.

[0008] WO 01/03802 discloses a composite filter comprising at least onenon-prebonded upstream tier and one non-prebonded downstream tier.However, as will be shown in detail below (FIG. 2), in this compositefilter a relatively high pressure drop across the composite filteroccurs. Further, also the service life time of this filter is low.

[0009] In view of this, the objective problem underlying the inventionis to provide a composite filter in which the pressure drop across thefilter is maintained low and which has a long service life time.

SUMMARY OF THE INVENTION

[0010] This objective problem is solved by a composite filter forfiltering a stream of ambient air comprising at least one non-prebonded,upstream tier and one non-prebonded downstream tier, wherein the ratioof absolute pore volume of upstream tier to downstream tier RAPV>2, andthe absolute projected fiber coverage of upstream tier and of downstreamtier APFC >95%.

[0011] Due to the parameters of this composite filter, the pressure dropacross the filter medium is kept low and the service life time of thefilter is increased.

[0012] Further, this invention enables to provide a composite filtermade up of at least two stacked tiers of filtration material bondedtogether to form a unitary stratified structure. The composition offiltration material in any given tier is preselected to perform adesired filtering function. For example, fine, (i.e., small diameter)and densely packed fibers can be selected to capture very small dustparticles such as those of about 5 micrometers and smaller.Additionally, electrostatically charged fibers can also be used to stoppassage of these and even smaller particles. Similarly, bulky, highlyporous media designed to have large dust holding capacity can beutilized to trap medium to large size dirt particles.

[0013] Since the composite filter of the invention comprises pre-bondedtiers, the bonding of at least one and preferably all of the tiers toform the unitary structure is begun only after the stacking of all thetiers of a particular desired composite filter structure has beencompleted. The resulting structure is a single body composed ofdifferent types of filtration material which appear as distinct strata.

[0014] In view of this, the stratified structure is formed by buildingup a stack of tiers of selected filtration materials. Because the tiersare non-prebonded, the components of each tier, that is, fibers,granules, etc., generally are laid loosely by mechanical or air-layingprocesses onto the layer lying below. Within each tier the compositionof filter material is largely uniform and there is a “fuzzy” interfacebetween the tiers.

[0015] Preferably, the composite filter of the above mentioned kindcomprises a ratio of average pore diameter of upstream to downstreamtier RPD in the range of 4<RPD<10.

[0016] Due to this ratio, the dust holding capacity of the upstream tieris greatly increased, such that the upstream tier acts as a pre filterfor the downstream tier without increasing the pressure drop across thecomposite filter.

[0017] Additionally but not exclusively, such composite filter maycomprise an average pore diameter of the upstream tier PDU, with PDU>60μm, preferably in the range 80 μm<PDU<200 μm.

[0018] All above discussed composite filters may comprise upstream tierswith a relative pore volume RPVU>94%, preferably RPVU>96%, an apparentdensity ADU<0.05 g/cm³, and a thickness D in the range of 0.5 mm<D<2.5mm. Choosing these parameters results in an upstream tier with therequired RAPV and APFC.

[0019] Further, this composite filters may also comprise downstreamtiers with a relative pore volume RPVD being smaller than RPVU, anapparent density ADD in the range of 0.07 g/cm³<ADD<0.14 g/cm³, and athickness D in the range of 0.1 mm<D<0.4 mm. Choosing these parametersresults in an downstream tier with the required RAPV and APFC.

[0020] Further, the upstream tier of any of the above discussedcomposite filter may preferably comprise fibers having a length in therange of 0.1 mm to 3.0 mm.

[0021] Due to such a structure, the upstream tier can be made bulkier toprovide greater dust holding capacity.

[0022] Preferably, the above discussed composite filters may comprise anupstream tier having a dust retention DR with respect to dust particleswith a diameter corresponding to the average pore diameter of thedownstream tier of DR>99%.

[0023] This feature avoids an clogging of the downstream tier, andtherefore, further maintains a low pressure drop across the filter andfurther increases the service life time of the composite filter.

[0024] Additionally but not exclusively, this effect can be increased ina composite filter in which the orientation of the fibers in flowdirection in the upstream tier is higher than in the downstream tier.Such structure further improves maintaining the pressure drop across thefilter.

[0025] The composite filters, as discussed above, may comprise anupstream tier of dry-laid, thermally bondable fusing, bicomponent ormonocomponent polymer fibers and a downstream tier of meltblown fibers.In this aspect a single tier is constituted of a single type of filtermedium, for example, 100% bicomponent polymer fibers, melt blown, staplefibers, or spunbond filaments.

[0026] Alternatively, the composite filter may comprise an upstream tierhaving a composition selected from the group consisting of 100 wt %bicomponent polymer fibers, a blend of at least about 10 wt %bicomponent polymer fibers with a complementary amount of naturalfibers, such as fluff pulp fibers or kokon fibers, staple fibers or amixture thereof, and a blend of at least about 10 wt % monocomponentpolymer thermally bondable fusing fibers with a complementary amount offluff pulp fibers, staple fibers or a mixture thereof.

[0027] In this aspect, a single tier is constituted by a blend of media,such as an air-laid, usually uniform blend of bicomponent polymer fibersand fluff pulp (FP) fibers.

[0028] Since it is also desirable to provide a stratified structure,adjacent tiers in a stack may have different compositions. Nonetheless,a composition of one tier can be repeated in a stack although at leastone tier of different composition should be present between the tiers ofsame composition.

[0029] This structure of the composite filter differs from that ofconventional multilayer filtration media which are formed by laminatinga plurality of individual filter medium layers that have each beenpre-bonded to form a self-supporting web prior to formation of themultilayer laminate.

[0030] Such unitary stratified structure provides a number ofsignificant advantages over conventional filter media. In one aspect,the unitary stratified structure can be made bulkier to provide greaterdust holding capacity than a laminate of individually, pre-bonded layershaving compositions corresponding respectively to the tiers of theunitary structure. This is because each portion of the conventionalfilter medium is compressed at least twice: once when the individuallayer is formed by bonding, and a second time when the individual layersare laminated to form the filter.

[0031] Preferably, the bicomponent polymer fibers of this structure mayhave a sheath of one polymer and a core of a different polymer having amelting point higher than the one polymer. The core may comprisepolypropylene and the sheath may comprise polyethylene.

[0032] Additionally, the core may be disposed eccentric relative to thesheath. In such a structure, the fibers will crimp with the result thatthe bulkiness of the tier is further increased.

[0033] Preferably and alternatively, the above discussed compositefilter may comprise an upstream tier further having fibers selected fromat least one of uncharged split film fibers, charged split film fibersand mixed electrostatic fibers.

[0034] Accordingly, the present invention now provides a compositefilter comprising at least two nonprebonded tiers each tierindependently comprising at least one filtration material and beingdistinct from the adjacent tier, in which the tiers are bonded togetherto form a unitary stratified structure having a first boundary surfaceadapted to receive particulates entrained in air and a second boundarysurface adapted to discharge filtered air, this composite filter showinga reduced pressure drop and a prolonged service life time.

[0035] All above discussed composite filters may be embodied in a vacuumcleaner bags, and more generally in vacuum filters. By “vacuum filter”is meant a filter structure intended to operate by passing a gas,preferably air, which entrains usually dry solid particles, through thestructure. The convention has been adopted in this application to referto the sides, tiers and layers of the structure in relation to thedirection of air flow. That is, the filter inlet side is “upstream” andthe filter discharge side is “downstream” for example. Occasionallyherein the terms “in front of” and “behind” have been used to denoterelative positions of structure elements as being upstream anddownstream respectively. Of course, there will be a pressure gradient,sometimes referred to as “pressure drop”, across the filter duringfiltration. Vacuum cleaners typically use bag shaped filters. Normally,the upstream side of a vacuum bag filter is the inside and thedownstream side is outside.

[0036] In addition to vacuum cleaner bags, the composite filter of theinvention can be utilized in applications such as heating ventilationand air conditioning (HVAC systems, vehicle cabin air filters, highefficiency (so-called “HEPA”) and clean room filters, emission controlbag house filters, respirators, surgical face masks and the like.Optionally, the composite filter can be used in such applications withan additional carbon fiber or particle-containing layer in series withthe composite filter of the invention, for example to absorb odors ortoxic contaminants. Moreover, certain applications, such as HEPA andclean room filters can employ additional layers in series with thecomposite filter of the invention, such as low porositypolytetrafluorethylene (PTFE) membrane laminated to a boundary surfaceof an appropriate unitary stratified structure, composite filter.

[0037] The present invention also provides a method of making acomposite filter of the above kind, comprising the steps of

[0038] (a) laying down a filtration material onto a support to form theupstream non-prebonded tier,

[0039] (b) depositing onto the upstream tier the downstreamnon-prebonded tier, and

[0040] (c) bonding the tiers to form a composite filter having a unitarystratified structure.

BRIEF DESCRIPTION OF THE FIGURES

[0041]FIG. 1 is a schematic diagram showing in cross section anembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of two tiers.

[0042]FIG. 2 is a diagram showing the pressure drop of the compositefilter in FIG. 1 and a composite prior art filter.

[0043]FIG. 3 is a schematic diagram showing in cross section anotherembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of three tiers.

[0044]FIG. 4 is a schematic diagram showing in cross section anotherembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of four tiers.

[0045]FIG. 5 is a schematic diagram showing in cross section anotherembodiment of the composite filter in accordance with the inventionhaving a unitary stratified structure of five tiers.

[0046]FIG. 6 is a schematic diagram showing in cross section anotherembodiment of the two-tiered composite filter of FIG. 1 in combinationwith a filter layer adjacent thereto.

[0047]FIG. 7 is a schematic diagram showing in cross section anotherembodiment of the three-tiered composite filter of FIG. 3 in combinationwith a filter layer adjacent thereto.

[0048]FIG. 8 is a schematic diagram showing in cross section anotherembodiment of the four-tiered composite filter of FIG. 4 in combinationwith a filter layer adjacent thereto.

[0049]FIG. 9 is a schematic diagram showing in cross section anotherembodiment of the five-tiered composite filter of FIG. 5 in combinationwith a filter layer adjacent thereto.

[0050]FIG. 10 is a schematic cross section diagram showing thetwo-tiered composite filter of FIG. 6 bonded to an adjacent filter layerwith an adhesive or ultrasonically bonded layer.

[0051]FIG. 11 is a schematic cross section diagram showing thethree-tiered composite filter of FIG. 7 bonded to an adjacent filterlayer with an adhesive or ultrasonically bonded layer.

[0052]FIG. 12 is a schematic cross section diagram showing thefour-tiered composite filter of FIG. 8 bonded to an adjacent filterlayer with an adhesive or ultrasonically bonded layer.

[0053]FIG. 13 is a schematic cross section diagram showing thefive-tiered composite filter of FIG. 9 bonded to an adjacent filterlayer with an adhesive or ultrasonically bonded layer.

[0054]FIG. 14 is a schematic diagram of an inline process for producinga composite filter according to a preferred embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0055] In the following, and before discussing explicitly discussing thepreferred embodiments of the invention, different filter materialcompositions which suitably used in the present invention are describedin greater detail:

[0056] Regarding the discussion below, DIN 44956-2 test has beenemployed to determine the increase in pressure drop of five differentexamples of vacuum cleaner bag constructions after dust loading withfine dust at the following levels: 0, 0.5, 1.0, 1.5, 2.0, and 2.5 grams.

[0057] Air Permeability after Fine Dust Loading Test: The dust loadingpart of the DIN 44956-2 is performed at 0.5 gram increments from 0 to2.5 g/(m²×s) on seven bags of each sample. However, the pressure dropvalues are not recorded again. The maximum sustainable air permeabilityvalues are then determined on the bags, which had the specified levelsof dust loading.

[0058] Standard Vacuum Cleaner Filter Bag Material:

[0059] This material, sometimes referred to as “standard paper” hastraditionally been used as a single ply in which it provides dustfiltration and containment, as well as the strength and abrasionresistance required of a vacuum cleaner bag. This material is also rigidenough to enable easy fabrication on standard bag manufacturingequipment. The paper is predominantly composed of unbleached wood pulpwith 6-7% of a synthetic fiber such as poly[ethylene terephthalate](PET) type polyester, and is produced by the wet laying process. Thestandard paper typically has a basis weight of about 30-80 g/m² andcommonly about 50 g/m². The PET fibers typically have a fineness of 1.7dtex and lengths of 6-10 mm. This paper has air permeability in therange of about 200-500 L/(m²×s) and an average pore size of about 30 mm.However, the efficiency as determined from the DIN 44956-2 Test is onlyabout 86%. Another characteristic is that the pores are quickly cloggedwith dust and the dust holding capacity is further limited-by the verythin paper thickness of only about 0.20 mm.

[0060] Spunbond Nonwoven:

[0061] A nonwoven of spunbond polymer fibers can be deployed as afiltration tier in the structure. The fibers can be of anyspunbond-capable polymer such as polyamides, polyesters or polyolefins.Basis weight of the spunbond nonwoven should be about 10-100 g/m² andpreferably about 3040 g/m². The spunbond nonwoven should have an airpermeability of about 500-10,000 L/(m²×s), and preferably about2,000-6,000 L/(m²×s) as measured by DIN 53887. The spunbond can also beelectrostatically charged.

[0062] Scrim or Supporting Fleece:

[0063] Scrim refers to a generally light basis weight, very open porouspaper or nonwoven web. Basis weight of the scrim is typically about10-30 g/m² ₁ and frequently about 13-17 g/m². The scrim, sometimesreferred to as a supporting fleece usually has air permeability of about500-10,000 L/(m²×s). It is primarily employed to protect other tiers orlayers from abrasion. The scrim can also filter the very largestparticles. The scrim, as well as any tier of the filter composite, canbe electrostatically charged provided the material has suitabledielectric properties.

[0064] Wet-Laid High Dust Capacity Material:

[0065] Wet-laid High Dust Capacity material, frequently referred toherein as “wet-laid capacity paper” is bulkier, thicker and morepermeable than the standard vacuum cleaner bag filter paper. It performsmultiple functions. These include resisting shock loading, filtering oflarge dirt particles, filtering a significant portion of small dustparticles, holding large amounts of particles while allowing air to flowthrough easily, thereby providing a low pressure drop at high particleloading which extends the life of the filter.

[0066] The wet-laid capacity paper usually comprises a fiber mixture ofwood pulp fibers and synthetic fibers. It typically contains up to about70% wood pulp and correspondingly more synthetic fiber, such as PET,than the standard paper described above. It has a greater thickness thanthe standard paper of about 0.32 mm at a typical basis weight of 50g/m². Pore size also is much greater, in that the average pore size canbe greater than 160 mm. Thus, the paper is able to hold much more dustin its pores before clogging up. Basis weight of the wet-laid capacitypaper typically is about 30-150 g/m² and preferably about 50-80 g/m².

[0067] The wet-laid capacity paper has a fine dust particle filtrationefficiency of about 66-67% as determined by the DIN 44956-2.Importantly, the wet-laid capacity paper has air permeability higherthan the standard filter paper. The permeability lower limit thuspreferably should be at least about 500 L/(m²×s), more preferably atleast about 1,000 L/(m²×s) and most preferably at least about 2,000L/(m²×s). The upper limit of permeability is defined to assure that thepaper filters and holds a major fraction of the dust particles largerthan about 10 mm. Consequently, a secondary high efficiency filtermedium positioned downstream is able to filter out and contain fineparticles much longer before showing indication of a substantialpressure drop increase across the filter. Accordingly, the airpermeability of the wet-laid capacity paper preferably should be at mostabout 8,000 L/(m²×S), more preferably at most about 5,000 L/(m²×s), andmost preferably at most about 4,000 L/(m²×s). It is thus seen that thewet-laid capacity paper is especially well designed as a multipurposefiltration tier to be positioned upstream of the secondary highefficiency filtration tier.

[0068] Dry-Laid High Dust Capacity Material:

[0069] Dry-laid high dust capacity material, sometimes referred toherein as “dry-laid capacity paper”, had not been used as a filter invacuum cleaner bags. Dry-laid paper is not formed from a water slurry,but is produced with air-laying technology and preferably by a “fluffpulp” process. Hydrogen-bonding which plays a large roll in attractingthe molecular chains together does not operate in the absence of water.Thus, at the same basis weight, dry-laid capacity paper, is usually muchthicker than standard paper and the wet-laid capacity paper. For atypical weight of 70 g/m², the thickness is 0.90 mm, for example.

[0070] The dry-laid capacity paper webs can be bonded primarily by twomethods. The first method is latex bonding in which the latex binder maybe applied from water-based dispersions. Saturation techniques such asspraying or dipping and squeezing (padder roll application) followed inboth cases by a drying and beat curing process can be used. The latexbinder may also be applied in discrete patterns such-as dots diamonds,cross hatches or wavy lines by gravure roll followed by drying andcuring.

[0071] The second method is thermal bonding, for example by utilizingbinder fibers. Binder fibers sometimes referred to herein as “thermallybondable fusing fibers” are defined by the Nonwoven Fabric Handbook,(1992 edition) as “Fibers with lower softening points than other fibersin the web. Upon the application of heat and pressure, these act as anadhesive.” These thermally bondable fusing fibers generally completelymelt at locations where sufficient heat and pressure are applied for theweb, thereby adhering the matrix fibers together at their cross-overpoints. Examples include co-polyester polymers which when heated adherea wide range of fibrous materials.

[0072] In a preferred embodiment thermal bonding can be accomplished byadding from at least 20% preferably up to 50% of a bicomponent (“B/C”)polymer fiber to the dry-laid web. Examples of B/C fibers include fiberswith a core of polypropylene (“PP”) and a sheath of more heat sensitivepolyethylene (“PE”). The term “heat sensitive” means that thermoplasticfibers soften and become sticky or heat fusible at a temperature of 3-5degrees C. below the melting point. The sheath polymer preferably shouldhave a melting point in the range of about 90-160 degrees C. and thecore polymer should have a higher melting point, preferably by at leastabout 5 degrees C. higher than that of the sheath polymer. For example,PE melts at 121 degrees C. and PP melts at 161-163 degrees C. This aidsin bonding the dry-laid web when it passes between the nip of a thermalcalendar or into a through-air oven by achieving thermally bonded fiberswith less beat and pressure to produce a less compacted, more open andbreathable structure. In a more preferred embodiment the core of thecore/sheath of the B/C fiber is located eccentric of the sheath. Themore that the core is located towards one side of the fiber the morelikely that the B/C fiber will crimp during the thermal bonding step,and thereby increase the bulk of the dry-laid capacity. This will, ofcourse, improve its dust holding capacity. Thus, in a still furtherpreferred embodiment the core and sheath are located side-by-side in theB/C fiber, and bonding is achieved with a through-air oven. A thermalcalendar, which would compress the web more than through-air bonding andis less preferred in this case. Other polymer combinations that may beused in core/sheath or side-by-side B/C fibers include PP withco-polyester, low melting polymers, and polyester with nylon 6. Thedry-laid high capacity tier can also be constituted essentiallycompletely by bicomponent fibers. Other variations of bicomponent fibersin addition to “sheath/core”, can be used, such as “side-by-side”,“islands in the sea” and “orange” embodiments disclosed in NonwovenTextiles, Jirsak, O., and Wadsworth, L. C., Carolina Academic Press,Durham, N.C., 1999, pp. 26-29.

[0073] Generally, the average pore size of dry-laid capacity isintermediate between the pore size of the standard paper and wet-laidcapacity paper The filtration efficiency as determined by the DIN44956-2 Test is approximately 80%. Dry-laid capacity paper should haveabout the same basis weight and the same permeability as the wet-laidcapacity paper described above, i.e., in the range of about 500-8,000L/(m²×s), preferably about 1,000-5,000 L/(m²×s) and most preferablyabout 2,000-4,000 L/(m²×s). It has excellent dust holding capacity andhas the advantage of being much more uniform in weight and thicknessthan the wet-laid papers.

[0074] Several preferred embodiments of dry-laid capacity paper arecontemplated. One is a latex bonded fluff pulp fiber composition. Thatis, the fibers comprising the paper consist essentially of fluff pulp.The term “fluff pulp” means a nonwoven component of the filter of thisinvention which is prepared by mechanically grinding rolls of pulp,i.e., fibrous cellulose material of wood or cotton, then aerodynamicallytransporting the pulp to web forming components of air laying or dryforming machines. A Wiley Mill can be used to grind the pulp. So-calledDan Web or M and J machines are used for dry forming. A fluff pulpcomponent and the dry-laid tiers of fluff pulp are isotropic and arethus characterized by random fiber orientation in the direction of allthree orthogonal dimensions. That is, they have a large portion offibers oriented away from the plane of the nonwoven web, andparticularly perpendicular to the plane, as compared tothree-dimensionally anisotropic nonwoven webs. Fibers of fluff pulputilized in this invention preferably are from about 0.5-5 mm long. Thefibers are held together by a latex binder. The binder can be appliedeither as powder or emulsion.

[0075] Binder is usually present in the dry-laid capacity paper in therange of about 10-30 wt % and preferably about 20-30 wt % binder solidsbased on weight of fibers.

[0076] Another preferred embodiment the dry-laid capacity papercomprises a thermally bonded blend of fluff pulp fibers and at least oneof “split film fibers” and bicomponent polymer fibers. More preferably,the blend of fluff pulp fibers comprises fluff pulp fibers andbicomponent polymer fibers.

[0077] Split Film Fibers:

[0078] Split film fibers are essentially flat, rectangular fibers whichmay be electrostatically charged before or after being incorporated intothe composite structure of the invention. The thickness of the splitfilm fibers may range from 2-100 micrometers, the width may range from 5micrometers to 500 micrometers, and the length may range from 0.5 to 15mm. However, the preferred dimensions of the split film fibers are athickness of about 5 to 20 micrometers, a width of about 15 to 60micrometers, and a length of about 0.5 to 8 mm.

[0079] The split film fibers of the invention are preferably made of apolyolefin, such as polypropylene. However, any polymer which issuitable for making fibers may be used for the split film fibers of thecomposite structures of the invention. Examples of suitable polymersinclude, but are not limited to, polyolefins like homopolymers andcopolymers of polyethylene, polyterephthalates, such as poly(ethyleneterephthalate) (PET), poly(butylene terephthalate) (PBT),poly(cyclohexyl-dimethylene terephthalate) (PCT), polycarbonate, andpolychlorotrifluoroethylene (PCTFE). Other suitable polymers includenylons, polyamides, polystyrenes, poly4-methylpentene-1,polymethylmethacrylates, polyurethanes, silicones, polyphenylenesulfides. The split film fibers may also comprise a mixture ofhomopolymers or copolymers. In the present application, the invention isexemplified with split film fibers made of polypropylene.

[0080] The use of PP polymers with various molecular weights andmorphologies in laminate film structures has been shown to produce filmswith a proper balance of mechanical properties and brittleness requiredto produce split film fibers. These PP split film fibers may also besubsequently given the desired level of crimp. All dimensions of thesplit film fibers may, of course, be varied during manufacture of thefibers.

[0081] One method for production of the split fibers, is disclosed inU.S. Pat. No. 4,178,157. Polypropylene is melted and extruded into afilm which is then blown into a large tube (balloon) into which ambientair is introduced or allowed to enter, in accordance with conventionalblow stretching technology. Inflating the balloon with air serves toquench the film and to bi-axially orient the molecular structure of thePP molecular chains, resulting in greater strength. The balloon is thencollapsed and the film is stretched between two or more pairs of rollersin which the film is held in the nip of two contacting rollers, with theapplication of varying amounts of pressure between the two contactingrollers. This results in an additional stretch in the machine directionwhich is accomplished by driving the second set of rollers at a fastersurface speed than the first set. The result is an even greatermolecular orientation to the film in the machine direction which willsubsequently become the long dimension of the split film fibers.

[0082] The film may be electrostatically charged before or after it hasbeen cooled down. Although various electrostatic charging techniques maybe employed to charge the film, two methods have been found to be mostpreferable. The first method involves passing the film about midway in agap of about 1.5 to 3 inches between two DC corona electrodes. Coronabars with emitter pins of metallic wire may be used in which one coronaelectrode has a positive DC voltage potential of about 20 to 30 kV andthe opposing electrode has a negative DC voltage of about 20 to 30 kV.

[0083] The second, preferred, method utilizes the electrostatic chargingtechnologies described in U.S. Pat. No. 5,401,446 (Wadsworth and Tsai,1995), which is referred to as Tantret(tm) Technique I and Technique II,which are further described herein. It has been found that Technique II,in which the film is suspended on insulated rollers as the film passesaround the inside circumference of two negatively charged metal shellswith a positive corona wire of each shell, imparts the highest voltagepotentials to the films. Generally, with Technique II, positive 1,000 to3,000 volts or more may be imparted to on one side of the films withsimilar magnitudes of negative volts on the other side of the chargedfilm. Technique 1, wherein films contact a metal roller with a DCvoltage of −1- to −10 kV and a wire having a DC voltage of +20 to +40 kVis placed from about 1 to 2 inches above the negatively biased rollerwith each side of the film being exposed in succession to thisroller/wire charging configuration, results in lower voltage potentialsas measured on the surfaces of the films. With Technique I, voltages of300 to 1,500 volts on the film surface with generally equal but oppositepolarities on each side are typically obtained. The higher surfacepotentials obtained by Technique II, however, have not been found toresult in better measurable filtration efficiencies of the webs madefrom the split film fibers. Therefore, and because it is easier to laceup and pass the film through the Technique I device, this method is nowpredominately used to charge the films prior to the splitting process.

[0084] The cooled and stretched film may be hot or coldelectrostatically charged. The film is then simultaneously stretched andsplit to narrow widths, typically up to about 50 micrometers. The split,flat filaments are then gathered into a tow which is crimped in acontrolled numbers of crimps per centimeter and then cut into thedesired staple length.

[0085] In a particularly preferred embodiment, the dry-laid high dustcapacity paper comprises a blend of all of fluff pulp fibers,bicomponent polymer fibers, and electrostatically charged split filmfibers. Preferably, the fluff pulp fibers will be present at about 5-85wt %, more preferably about 10-70 wt %, and most preferably about 40 wt%, the bicomponent fibers at about 10-60 wt %, more preferably about10-30 wt % and most preferably about 20 wt %, and the electrostaticallycharged split film fibers at about 20-80 wt %, and more preferably about40 wt %. This dry-laid high dust capacity may be thermally bonded,preferably at a high temperature of 90-160 degrees C., more preferably,at a temperature lower than 110 degrees C. and most preferably at about90 degrees C.

[0086] Mixed Electrostatic Fibers:

[0087] Other preferred embodiments of the dry-laid capacity papercomprises a thermally bonded paper with 100% “mixed electrostaticfibers”, a blend of 20-80% mixed electrostatic fibers and 20-80% B/Cfibers, and a blend of 20-80% mixed electrostatic fibers, 110-70% fluffpulp and 10-70% B/C fibers. “Mixed electrostatic fiber” filters are madeby blending fibers with widely different triboelectric properties andrubbing them against each other or against the metal parts of machines,such as wires on carding cylinders during carding. This makes one of thetypes of fibers more positively or negatively charged with respect tothe other type of fiber, and enhances the coulombic attraction for dustparticles. The production of filters with these types of mixedelectrostatic fibers is taught in U.S. Pat. No. 5,470,485 and inEuropean Patent Application EP 0 246 811.

[0088] In U.S. Pat. No. 5,470,485, the filter material consists of ablend of (I) polyolefin fibers and (II) polyacrylonitrile fibers. Thefibers (I) are bicomponent PP/PE fibers of the core/sheath orside-by-side type. The fibers 11 are “halogen free”. The (I) fibers alsohave some “halogen-substituted polyolefins”: whereas, the acrylonitrilefibers have no halogen. The patent notes that the fibers must bethoroughly washed with nonionic detergent, with alkali, or solvent andthen well rinsed before being mixed together so that they do not haveany lubricants or antistatic agents. Although the patent teaches thatthe fiber mat produced should be needle-punched, these fibers could alsobe cut to lengths of 5-20 mm and mixed with similar length bicomponentthermal binder fibers and also with the possible addition of fluff pulpso that dry-laid thermally bonded paper can be utilized in thisinvention.

[0089] EP 0 246 811 describes the triboelectric effect of rubbing twodifferent types of fibers together. It teaches using similar types offibers as U.S. Pat. No. 5,470,485, except that the —CN groups of thepolyacrylonitrile fibers may be substituted by halogen (preferablyfluorine or chlorine). After a sufficient amount of substitution of —CNby —Cl groups, the fiber may be referred to as a “modacrylic” if thecopolymer comprises from 35 to 85% weight percent acrylonitrile units.EP 0 246 811 teaches that the ratio of polyolefin to substitutedacrylonitrile (preferably modacrylic) may range from 30:70 to 80:20 bysurface area, and more preferably from 40:60 to 70:30. Similarly, U.S.Pat. No. 5,470,485 teaches that the ratio of polyolefin topolyacrylonitrile fibers is in the range of 30:70 to 80:20, relative toa surface of the filter material. Thus, these ranges of ratios ofpolyolefin to acrylic or modacrylic fibers may be used in the abovestated proportions in the dry-laid thermally bonded capacity paper.

[0090] Meltblown Fleece:

[0091] A synthetic polymer fiber meltblown fleece can optionally bedeployed as an tier between a multipurpose tier and a high efficiencyfiltration tier. The meltblown fleece tier increases overall filtrationefficiency by capturing some particles passed by the multipurposefiltration tier. The meltblown fleece tier also optionally can beelectrostatically charged to assist in filtering fine dust particles.Inclusion of a meltblown fleece tier involves an increase in pressuredrop at given dust loading as compared to composites not having ameltblown fleece ter.

[0092] The meltblown fleece preferably has a basis weight of about 10-50g/m² and air permeability of about 100-1,500 L/(m²×s).

[0093] High Bulk Meltblown Nonwoven:

[0094] Another discovery from recent research to develop improved vacuumcleaner bags was the development of a high bulk MB web or tier whichcould be used upstream of a filtration grade MB fleece as a pre-filterin place of the wet-laid capacity paper or dry-laid capacity paper. Thehigh bulk MB pre-filter can be made in a meltblowing process usingchilled quench air with a temperature of about 10 degrees C. Incontrast, conventional MB normally uses room air at an ambienttemperature of 35-45 degrees C. Also the collecting distance from the MBdie exit to the web take-up conveyer is increased to 400-600 mm in thehigh bulk MB process. The distance normally is about 200 mm for regularMB production. Additionally, high bulk MB nonwoven is made by using alower temperature attenuation air temperature of about 215-235 degreesC. instead of the normal attenuation air temperature of 280-290 degreesC., and a lower MB melt temperature of about 200-225 degrees C. comparedto 260-280 degrees C. for filtration grade MB production. The colderquench air, lower attenuation air temperature, lower melt temperatureand the longer collecting distance cool down the MB filaments more.Removing beat results in less draw down of the filaments, and hence, inlarger fiber diameters than would be found in typical filtration gradeMB webs. The cooler filaments are much less likely to thermally fusetogether when deposited onto the collector. Thus, the High BulkMeltblown nonwoven would have more open area. Even with a basis weightof 120 g/m², the air permeability of the High Bulk Meltblown nonwoven is806 L/(m²×s). By contrast, a much-lighter (e.g., 22 g/m²) filtrationgrade MB PP web had a maximum air permeability of only 450 L/(m²×s). Thefiltration efficiency of the High Bulk MB nonwoven as determined by theDIN 44956-2 Test was 98%. When the two were put together with the HighBulk MB nonwoven on the inside of the bag, the air permeability wasstill 295 L/(m²×s), and the filtration efficiency of the pair was 99.8%.The high bulk meltblown nonwoven can be uncharged, or optionallyelectrostatically charged provided that the nonwoven is of materialhaving suitable dielectric properties.

[0095] High Bulk MB nonwoven of this invention should be distinguishedfrom “filtration grade MB” which also is employed in the multitiervacuum filter structure of this disclosure. Filtration grade MB web is aconventional meltblown nonwoven generally characterized by a low basisweight typically of about 22 g/m², and a small pore size. Additionaltypical characteristics of filtration grade MB nonwoven of polypropyleneare shown in Table 1. A preferred high bulk MB nonwoven of polypropyleneoptimally includes about 5-20 wt % ethylene vinyl acetate. Filtrationgrade MB nonwoven has generally high dust removal efficiency, i.e.,greater than about 99%. TABLE I More Most Preferred Preferred PreferredFiltration Grade MB PP Weight g/m² 5-100 10-50 25 Thickness, mm 0.10-20.10-1 0.26 Air Permeability, L/(m² × s) 100-5,000 100-2,000 450 TensileStrength, MD, N 0.5-15 1.0-10 3.7 Tensile Strength, CD, N 0.5-15 1.0-103.2 Fiber Diameter, mm 1-15 1-5 2-3 High Bulk MB PP Weight, g/m² 30-18060-120 80 Thickness, min 0.3-3 0.5-2 1.4 Air permeability, L/(m² × s)300-8,000 600-3,000 2,000 Tensile Strength, MD, N 1.0-30 2-20 10 TensileStrength, CD, N 1.0-30 2-20 9.2 Fiber Diameter, mm 5-20 10-15 10-12

[0096] High Bulk MB nonwoven is similar in filter efficiency to dry-laidand wet-laid capacity papers mentioned above. Thus, High Bulk MBnonwoven is well-adapted to remove large quantities of large dustparticles and to hold large amounts of dust. Accordingly, High Bulk MBnonwoven tier is suited for placement upstream of and as a pre-filterfor a filtration grade MB tier in a vacuum filter structure of thisinvention.

[0097] Spunblown (Modular) Nonwoven:

[0098] A new type of meltblowing technology described in Ward, G.,Nonwovens World, Summer 1998, pp. 3740 is available to produce aSpunblown (Modular) Nonwoven suitable for use as a coarse filter tier inthe present invention. Optionally, the Spunblown Nonwoven can beutilized as a filtration grade meltblown fleece tier as called for inthe novel structure. Specifications of the Spunblown (Modular) Nonwovenare presented in Table II.

[0099] The process for making the Spunblown (Modular) Nonwoven isgenerally a meltblown procedure with a more rugged modular die and usingcolder attenuation air. These conditions produce a coarse meltblown webwith higher strength and air permeability at comparable basis weight ofconventional meltblown webs.

[0100] Microdenier Spunbond Nonwoven:

[0101] A spunbond (“SB”) nonwoven, occasionally referred to herein asmicrodenier spunbond can also be utilized in this invention in the sameway as the coarse filter tier or the filtration grade meltblown fleecetier previously mentioned. Specifications of microdenier spunbond arelisted in Table II. Microdenier spunbond is particularly characterizedby filaments of less than 12 mm diameter which corresponds to 0.10denier for polypropylene. In comparison, conventional SB webs fordisposables typically have filament diameters which average 20 mm.Microdenier spunbond can be obtained from Reifenhauser GmbH (ReicofilII), Koby Steel, Ltd., (Kobe-Kodoshi Spunbond Technology) and AsonEngineering, Inc. (Ason Spunbond Technology). TABLE II More MostPreferred Preferred Preferred Spunblown (Modular) Weight g/m² 10-15010-50 28 Thickness, mm 0.20-2 0.20-1.5 0.79 Air permeability, L/(m² × s)200-4,000 300-3,000 1,200 Tensile Strength, MD, N 10-60 15-40 43 TensileStrength, CD, N 10-50 12-30 32 Fiber Diameter, micrometer 0.6-20 2-102-4 microdenier spunbond PP (Ason, Kobe-Kodoshi, Reicofil III) Weight,g/m² 10-50 20-30 17 Thickness, mm 0.10-0.6 0.15-0.5 0.25 Airpermeability, L/(m² × s) 1,000-10,000 2,000-6,000 2,500 TensileStrength, MD, N 10-100 20-80 50 Tensile Strength, CD, N 10-80 10-60 40Fiber Diameter, micrometer 4-18 6-12 10

[0102] Preferred Embodiments:

[0103] Representative products according to the present invention areillustrated schematically in FIGS. 1, 3-13, and described in greaterdetail as follows. In the figures, air flow direction is indicated byarrow A.

[0104] In FIG. 1, a unitary composite filter 36 made from two tiers isdepicted. The upstream (dirty air side) tier 37 is a Dry-Laid FPCapacity tier with the broadest weight of 10-150 g/m², typical weightrange of 20-80 g/m² and with a preferred weight of 75 g/m². The FP layer37 has different blends of pulp fibers and bicomponent (B/C) fibers. Thebicomponent fibers comprise 60% PE and 40% PP. The downstream tier 38 isa high efficiency MB component with a weight of 5-100 g/m², preferably24 g/m². Notably, the independently composed tiers 37 and 38 meet atinterface 36A. This interface is different from that in a laminate oftwo pre-bonded layers in a multilayer composite. Due to the fact thatformation of a pre-bonded layer is not needed to produce the structure36, at least one of tiers 37 and 38 can be sufficiently flimsy that itcould not be formed into a free standing web to be incorporated as alayer in a conventional multilayer composite.

[0105] The upstream tier has an absolute pore volume of 21.4 cm³/g, thedownstream tier of 7.7 cm³/g, resulting in an ratio of absolute porevolume RAPV=2.78. The absolute projected fiber coverage, i.e. the unitarea which is covered by fibers when perpendicularly looking at thetier, of the upstream tier APFC is 97.7%. APFC of the downstream tier is99.3%.

[0106] To optimize the dust holding capacity, a ratio of average porediameter of upstream tier to downstream tier of 6.21 is realized, theaverage pore diameter of the upstream tier being 87 micrometer, theaverage pore size of the downstream tier being 14 micrometer.

[0107] In order to obtain the above RAPV and APFC values, the upstreamtier comprises a thickness of 1.7 mm, an apparent density of 0.044g/cm³, and a relative pore volume of 94.4%. The downstream tiercomprises a thickness of 0.21 mm, an apparent density of 0.11 g/cm³, anda relative pore volume of 87.4%. It is understood that these values areexemplary only; in particular the above RAPV and APFC values can also beobtained with different thickness, apparent density, and relative porevolume.

[0108]FIG. 2 illustrates the highly improved pressure drop across thefilter depending on the amount of dust filtered by the composite filter.The upper curve shows the composite filter with the characteristicsdiscussed above. The lower curve shows a prior art filter, consisting ofa spunbond as upstream tier and a meltblown as downstream tier. Theprior art upstream tier has an absolute pore volume of 6.9 cm³/g, thedownstream tier of 8.1 cm³ μg, resulting in an ratio of absolute porevolume RAPV=0.85. The absolute projected fiber coverage of the upstreamtier is APFC 69.3%. APFC of the downstream tier is 92.3%.

[0109] A further embodiment (not shown) has the same structure as theembodiment shown in FIG. 1. This embodiment, however, comprises anupstream tier in form of a Dry-Laid FP Capacity tier with a weight of 50g/m². The FP layer has different blends of pulp fibers and bicomponent(B/C) fibers. The bicomponent fibers comprise 60% PE and 40% PP. Thedownstream tier is a high efficiency MB component with a weight of 24g/m². The upstream tier has an absolute pore volume of 22.7 cm³/g, thedownstream tier of 7.7 cm³/g, resulting in an ratio of absolute porevolume RAPV=2.95. The absolute projected fiber coverage of the upstreamtier is APFC 99.9%. APFC of the downstream tier is 99.3%.

[0110] To optimize the dust holding capacity, a ratio of average porediameter of upstream tier to downstream tier of 5.93 is realized, theaverage pore diameter of the upstream tier being 83 micrometer, theaverage pore size of the downstream tier being 14 micrometer.

[0111] In order to obtain the RAPV and APFC values, the upstream tiercomprises a thickness of 1.2 mm, an apparent density of 0.042 g/cm³, anda relative pore volume of 94.7%. The downstream tier comprises athickness of 0.21 mm, an apparent density of 0.11 g/cm³, and a relativepore volume of 87.4%.

[0112] In a further embodiment (not shown) the upstream tier comprisessplit film fibers and “mixed electrostatic fibers.” Split film fibersand “mixed electrostatic fibers” are not used in all variations of theupstream tier, but at least 10% and preferably at least 20% B/C fibersor other types of thermally bondable fusing fibers should be used toachieve adequate thermal bonding. Generally, at least 10% and preferablyat least 20% pulp fibers are used for enhanced cover and filtrationefficiency. The tier can be free of B/C fibers or other types ofthermally bondable fusing fibers if latex binder is used.

[0113]FIG. 3 depicts a unitary composite filter 39 composed of threetiers. The first tier 40 is a coarse drylaid component made of 100% B/Cfibers. It mainly serves as a pre-filter and protects downstream filtermaterial. The broadest weight range is 10-100 g/m² with a typical weightrange of 20-80 g/m², and a preferred weight of 50 g/m². The upstreamtier 41 is a Dry-Laid FP Capacity component as discussed in the aboveembodiments. The downstream tier 42 consists of high filtrationefficiency MB media or other ultrafine fiber diameter materials such asSpunBlown Modular or Microdenier Spunbond.

[0114]FIG. 4 is a diagram of a unitary composite filter 43 made fromfour tiers of material. The first tier 44 is composed of Dry-Laid FP of100% B/C fibers. The broadest weight range is from 10-100 g/m², typicalweight is from 20-80 g/m² and the target weight is 50 g/m². The upstreamtier 45 is a Dry-Laid FP Capacity tier as discussed in the aboveembodiments. Alternatively, tier 45 may contain at least 10% andpreferably at least 20% B/C fibers, 10% and preferably at least 20% pulpfibers, and may contain varying amounts of charged or uncharged splitfilm fibers. It may contain varying amounts of “mixed electrostaticfibers”. At least 10% and preferably at least 20% BIC fibers or othertypes of thermally bondable fusing fibers should be used to achieveadequate thermal bonding. Generally, at least 10% and preferably atleast 20% pulp fibers are used for enhanced cover and filtrationefficiency. The tier can be free of B/C fibers or other types ofthermally bondable fusing fibers if latex binder is used. The downstreamtier 46 contains MB filter media as discussed with respect to the aboveembodiments. The outer tier 47 is a Dry-Laid FP composed of air-laidpulp and B/C fibers.

[0115]FIG. 5 is a diagram of a unitary composite filter 48 made fromfive tiers of material. The first tier 49 is composed of Dry-Laid FP of100% B/C fibers. The broadest weight range is from 10-100 g/m², typicalweight is from 20-80 g/m² and the target weight is 50 g/m². The upstreamtier 50 is a Dry-Laid FP Capacity component as discussed above.Component 51 contains carbon granules or carbon fibers to absorb odorsand to remove pollutant and toxic gases from the air. Component 52 is ahigh filtration efficiency MB as discussed with respect to the aboveembodiments. Component 53 is a Dry-Laid FP composed of air-laid pulp andB/C fibers.

[0116]FIG. 6 depicts a unitary composite filter 54 of the sameconstruction as shown in FIG. 1, composed of two tiers 55, 56, bonded toa supporting outer layer 57 consisting of a paper, scrim or nonwovenwith a weight ranging from 10-100 g/m².

[0117]FIG. 7 depicts a unitary composite filter 58 of the sameconstruction as shown in FIG. 3, composed of three tiers 59, 60 and 61,bonded to an outer layer 62 consisting of a paper, scrim or nonwovenwith a weight ranging from 10-100 g/m².

[0118]FIG. 8 depicts a unitary composite filter 63 of the sameconstruction as FIG. 4, composed of four tiers 64-67, bonded to an outerlayer 68 consisting of a paper, scrim or nonwoven with a weight of10-100 g/m².

[0119]FIG. 9 depicts a unitary composite filter 69 of the sameconstruction as FIG. 5, composed of five tiers 71-75, bonded to an outerlayer 76 consisting of a paper, scrim or nonwoven with a weight of10-100 g/m².

[0120]FIG. 10 depicts a laminate of unitary composite filter 77 of thesame construction as shown in FIG. 1, composed of two tiers 78, 79,bonded to a supporting outer layer 81 consisting of a paper, scrim ornonwoven with a weight ranging from 10-100 g/m², wherein the outer layeris bonded by glue or an adhesive 80, in which the latter could be alatex binder or a hot melt adhesive.

[0121]FIG. 11 depicts a laminate of unitary composite filter 82 of thesame construction as shown in FIG. 3, composed of three tiers 83-85, toan outer layer 87 consisting of a paper, scrim or nonwoven with a weightranging from 10-100 g/m², wherein the outer layer is bonded by glue oran adhesive 86.

[0122]FIG. 12 depicts a laminate of unitary composite filter 87A of thesame construction as FIG. 4, composed of four tiers 88-91, to an outerlayer 93 consisting of a paper, scrim or nonwoven with a weight of10-100 91 mA², wherein the outer layer is bonded by glue or an adhesive92.

[0123]FIG. 13 depicts a laminate of unitary composite filter 94 of thesame construction as FIG. 5, composed of five tiers 95-99, to an outerlayer 101 consisting of a paper, scrim, or nonwoven with a weight of10-100 g/mA², wherein the outer layer is bonded by glue or an adhesive100.

[0124] Where bonding between layers is indicated in embodiments of FIGS.10-13, conventional interlayer bonding methods, such as ultrasonicbonding can be used in place of or in conjunction with glue/adhesivebonding mentioned above.

[0125] A preferred process for producing an embodiment of the novelcomposite filter comprising a unitary stratified structure of MB and FPcompositions is shown in FIG. 14. The illustrated process provides aproduct laminated to a scrim, paper or nonwoven to facilitate handling,pleating or packaging. It is also possible to provide an unlaminatedcomposite filter by replacing the scrim, paper or nonwoven with asupporting conveyor to carry the non-prebonded tiers through theprocess. The final unitary composite filter consists of at leasttwo-tiers, although each tier may contain more than one type of fiber orother materials as discussed above, and generally consists of three tofive tiers, which are thermally or latex bonded. The electrostaticcharging of the composite filter is preferably done in-line by theTantret “cold” electrostatic charging process, although MB fibers may be“hot” charged in-line upon exiting the MB die. Also, split film fibers,which were electrostatically charged during their production, may beintroduced by the FP applicators. Furthermore, “mixed electrostaticfibers” which have opposite polarities after rubbing against each otherdue to different triboelectric properties may be incorporated into thecomposite by the FP applicators.

[0126] Now referring to FIG. 14, an optional unwind I is located at thestarting end of the line to allow for the feeding in of an optionalsupport layer 2, which may be a scrim, paper or nonwoven. Components 1,2, 4 and 5 are optional in that the inventive unitary composite filteris laminated to a scrim, paper or nonwoven only to facilitate handling,pleating or packaging. A conveyor belt 3 runs the entire length of theline; however, it may also be separated into shorter sections with oneconveyer section feeding the assembly of tiers onto the next sections asrequired in the process. Also at the starting end of the line there isan optional adhesive applicator 4 for dispensing an adhesive 5 in theform of glue or hot melt adhesive. This adhesive application station canbe utilized when it is desired to in-line laminate a supporting layer tothe unitary stratified structure of the novel composite. However, itshould be noted that applicator 4 is not intended for pre-bonding tierswithin the stratified structure.

[0127] Next, as shown in FIG. 14, there are at least one, and preferablytwo, FP applicator units 6 and 8. The primary function of the FPapplicator units at the beginning of the line is to produce and depositdry-laid tiers 7 and 9 onto the optional adhesive tier 5, or onto theconveyor belt 3 if the optional support layer 2 and adhesive 5 are notused. The dry-laid tiers 7 and 9 may be the same or have differentcompositions and properties to meet the requirements of the end product.In any respect, the role of tiers 7 and 9 is primarily to support andprotect the MB or related filter media tiers 12 and 14. In theillustrated embodiment, the FP tiers 7 and 9 are primarily composed of“pulp” and bicomponent (B/C) fibers. Different types of B/C fibers maybe used as described above. For example, a preferred type has a core ofhigher melting point fibir such as PP and a sheath of lower meltingpoint fiber such as PE. Other preferred compositions of “pulp” and B/Ccore sheath PP/PE are 50% “pulp”/50% B/C fibers in tier 7 and 25% “pulp”75% B/C fibers in tier 9. If latex binder is not applied in section 23,then at least 20% B/C fibers or other types of thermal binder fibersshould be used. On the other hand, if latex bind is subsequently appliedin sections 23 and 27, then 100% “pulp” fiber can be applied by FPapplicators heads 6 and 8. Also, it is possible to apply 100% B/C fibersfrom FP applicator 6 or applicator 8, or from both applicator heads 6and 8.

[0128] In additional embodiments, instead of 100% B/C fibers,monocomponent regular staple fibers of PP, PET, polyamide and otherfibers can be substituted for up to 80% of the B/C or thermal bondingfibers that may be applied by any of the FP application heads 6, 8, 15,18, and 20. Many types of thermally bondable fusing fibers whichcompletely melt and are also known as “melt fibers” also can be used inplace of the B/C fibers, except in dry-laid tier components where 100%B/C fiber would be used.

[0129]FIG. 14 further illustrates optional compactor 10 which decreasesthe thickness of the web and increases fiber-to-fiber adhesion of FPtiers 7 and 9. It should be noted that the extensive pre-bondingtypically employed to separately produce the layers is not the objectiveof this optional compacting step utilized in this inventive in-lineprocess. The compactor 10 may be a calender, which may or may not beheated. The MB or related filter media 12 and 14 may be deposited by oneor more MB dies 11 and 13 onto the FP tiers 7 and 9. The primaryfunction of the MB component is to serve as a high efficiency filter,that is, to remove small percentages of small size (less than about 5micrometers) particles. The specifications of filtration grade MB mediaand related ultrafine fiber diameter types of filter, media are given inTable I.

[0130] The process can include at least one or more MB dies 11 and/orone or more related fine denier, (ultrafine fiber diameter) fiberapplicators 13, designated as X. For example, if two identical MB unitsare utilized, then units 11 and 13 will be the same. Other variationscontemplated to come within the breadth of this invention include havingthe first unit as a SpunBlown (Modular) or Microdenier Spunbond (SB)system first to form a filter gradient of coarser to finer highefficiency filters. Another contemplated variation is for one or moreSpunBlown (Modular) or Microdenier SB systems to be used in tandem.Still another variation is to use a Microdenier SB first followed by aSpunBlown system.

[0131] The next equipment component shown in FIG. 14 is another FPapplicator 15, which deposits an FP web on top of tier 14 (or on tier 12if a second MB tier 13 is not included). Then the non-prebonded assemblyof tiers with tier 16 uppermost travels through another optionalcompactor 17. Next the intermediate product is conveyed beneath one ormore additional FP units 18 and 20. FP applicator heads 15 and 18incorporate the Dry-Laid Capacity tier into the structure. FP applicator20 is primarily designed to produce very open (i.e. bulky) FP primarilyfor dust holding capacity rather than as a filter. The very open FP tier21 preferably is produced from 100% bicomponent B/C fiber or blends ofB/C with ratios of B/C to “pulp” characterized as being higher than isnormally used to produce coarse pre-filter FP webs. Either or both FPtiers 16 and 19 can also contain split film fibers and “mixedelectrostatic fibers”. If no B/C fibers or other types of thermalbonding fibers are used in FP tiers 16 and 19, then latex binder shouldbe applied at units 23 and 27 to bond the tiers. If B/C fibers or othertypes of thermal bonding fibers are incorporated in either of FPapplicator heads 15 and 18, then latex binder still can also be appliedat units 23 and 27.

[0132] The intermediate product with uppermost tier 21 then travelsthrough another compactor 22 and thence through a section of theproduction line where the previously loose, unbound tiers are subjectedto one or more binding process steps that are cumulatively effective toform the unitary stratified structure of the composite filter.Preferably, all of the filter components that will be incorporated intothe unitary stratified structure are incorporated in the intermediateproduct at this stage prior to binding the tiers together.

[0133] With further reference to FIG. 14, it is seen that the bindingsteps take place beginning in, the illustrated embodiment with a latexbinder 24 being applied by applicator 23. The latex can be sprayed froma liquid dispersion or emulsion, applied by kiss roll or gravureapplication, or sprayed as a dry powder onto the substrate and thenthermally fused or bonded thereto. The latex also serves as a sealant inthat it minimizes dust that can emanate from outside surfaces of the FPtier. After adding latex binder at 23, the intermediate product travelsthrough a heating unit 25 which dries and cures the latex binder to bondthe composite. The heating unit can be a heated calender, or aninfrared, microwave, or convection oven. A combination of these can alsobe used. A through-air oven is preferred. If B/C fibers or other typesof thermally bondable fusing fibers are present in the intermediateproduct, then ovens 25 and 29 can serve to thermally fuse such fibers tocontinue the bonding and formation of the unitary structure.

[0134] From the oven 25, the intermediate product is cooled by system26, and then a second latex binder is applied at 27. As illustrated, thepath of travel and spraying unit 27 are positioned to apply latex binderto the side opposite the first application. The intermediate productcontaining the second latex binder 28 then passes through a secondthrough-air oven 29 and through another cooling section 30. Next, thefully bonded composite film having a unitary stratified structure ischarge in cold electrostatic charging station 31, preferably, a TantretJ system. Finally, the composite film 32 is slit to desired width ormultiple of widths on shiter 33 and rolled up by the winder 34. Althoughelectrostatic charging is illustrated to take place toward the end ofthe process, it is contemplated that charging at a stage prior t6application of latex binder can be performed, provided that the binderand the subsequent procedural steps do not significantly drain thecharge from the intermediate product.

1. A composite filter for filtering a stream of ambient air comprisingat least one non-prebonded upstream tier and one non-prebondeddownstream tier, wherein the ratio of absolute pore volume of upstreamtier to downstream tier RAPV>2, and the absolute projected fibercoverage of upstream tier and of downstream tier APFC>95%.
 2. Thecomposite filter of claim 1, wherein the ratio of average pore diameterof upstream to downstream tier RPD is4<RPD<10.
 3. The composite filterof claim 2, wherein the average pore diameter of the upstream tierPDU>60 μm, preferably 80 μm<PDU<200 μm.
 4. The composite filter of anyone of claims 1 to 3, wherein the upstream tier comprises a relativepore volume RPVU>94%, preferably RPVU>96%, an apparent density ADU<0.05g/cm³, and a thickness D in the range of 0.5 mm<D<2.5 mm.
 5. Thecomposite filter of claim 4, wherein the downstream tier comprises arelative pore volume RPVD being smaller than RPVU, an apparent densityADD in the range of 0.07 g/cm³<ADD<0.14 g/cm³, and a thickness D in therange of 0.1 mm<D<0.4 mm.
 6. The composite filter of any one of claims 1to 5, wherein the upstream tier comprises fibers having a length in therange of 0.1 mm to 3.0 mm.
 7. The composite filter of claim 6, whereinthe orientation of the fibers in flow direction in the upstream tier ishigher than in the downstream tier.
 8. The composite filter of any oneof claims 1 to 7, wherein the upstream tier comprises a dust retentionDR with respect to dust particles with a diameter corresponding to theaverage pore diameter of the downstream tier of DR>99%.
 9. The compositefilter of any one of claims 1 to 8, wherein the upstream tier comprisesdry-laid, thermally bondable fusing, bicomponent or monocomponentpolymer fibers and the downstream tier comprises meltblown fibers. 10.The composite filter of claim 9, wherein the upstream tier has acomposition selected from the group consisting of 100 wt % bicomponentpolymer fibers, a blend of at least about 10 wt % bicomponent polymerfibers with a complementary amount of natural fibers, staple fibers or amixture thereof, and a blend of at least about 10 wt % monocomponentpolymer thermally bondable fusing fibers with a complementary amount offluff pulp fibers, staple fibers or a mixture thereof.
 11. The compositefilter of claim 10, wherein the bicomponent polymer fibers have a sheathof one polymer and a core of a different polymer having a melting pointhigher than the one polymer.
 12. The composite filter of claim 11,wherein the core is polypropylene and the sheath is polyethylene. 13.The composite filter of claim 12, wherein the core is disposed eccentricrelative to the sheath.
 14. The composite filter of claim 9, wherein theupstream tier further comprises fibers selected from at least one ofuncharged split film fibers, charged split film fibers and mixedelectrostatic fibers.
 15. A vacuum cleaner bag comprising a compositefilter in accordance with any one of the preceding claims.
 16. A methodof making a composite filter in accordance with anyone-of claims 1 to14, comprising the steps of (a) laying down a filtration material onto asupport to form the upstream non-prebonded tier, (b) depositing onto theupstream tier the downstream non-prebonded tier, and (c) bonding thetiers to form a composite filter having a unitary stratified structure.17. A composite filter produced by the method of claim
 16. 18. A vacuumcleaner bag comprising a composite filter produced by the method ofclaim 16.